1. Field of the Invention
The present invention relates to lithography systems and device manufacturing methods.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
A double exposure technique can be used to improve resolution of optical exposure systems by exposing lower resolution patterns interstitially using first and second images, which can effectively double the resolution capability of an optical system. Extremely high resolution patterns are deconvolved into two lower resolution patterns (e.g., the first and second images), which can be exposed interstitially on the substrate.
One way to perform double exposure is to expose the first image, remove the wafer from the exposure tool, develop and process the wafer, recoat the wafer with a resist, reload the wafer in the exposure tool, and expose the wafer with the second image.
Another way to perform double exposure is to use a contrast enhancing layer on top of the resist layer. This contrast enhancing layer increases resolution of patterned images and prevents the first image from exposing resist in the area where the second image will be placed. The contrast enhancing layer is removed between exposures and reformed. Two passes through the exposure system are performed, along with substrate alignment between passes. Thus, removing and reapplying the contrast enhancing layer can dramatically decrease throughput, and may cause misalignment between patterns formed by the first and second images.
Therefore, what is needed is a system and method that provide a contrast enhancing layer that does not need to be removed and reapplied between exposures in a multi-exposure process.